Why do I get this error?

you are missing a curly brace, the correct code is

function getRandomNumber( lower, upper) { return Math.floor(Math.random() * (upper - lower + 1) + lower); }

It looks like the problem is one of the parentheses isn't closed. You've got:

(lower, upper) - good!

(Math.random() <---notice the opening parenthesis here before the 'M' in Math. This doesn't have a closing parenthesis.

(upper - lower + 1) - good!

not sure if the formatting got lost when you posted it, but following the formatting practices in the course, it should look something like this:

function getRandomNumber(lower, upper) { return Math.floor(Math.random() * (upper - lower + 1) + lower); }

also, assuming you're using Workspaces, if you position the cursor next to one of your braces (curly or parenthesis), the brace/parenthesis will appear red if it's not closed. if it is closed, it will show highlighted in yellow, and the other associated brace or parenthesis that opens or closes it will also be highlighted.

(code <--cursor next to the parenthesis would cause it to appear in red (code) <--cursor next to either parenthesis would cause both parentheses to highlight yellow

I've found this, and judicious use of the JavaScript console extremely helpful in identifying where I've missed something in my code.

function getRandomNumber( lower, upper) { return Math.floor(Math.random() * (upper - lower + 1) + lower ; }

Your code above is missing a closing parenthesis after "lower" argument:

The code below should work fine.

function getRandomNumber( lower, upper) { return Math.floor(Math.random() * (upper - lower + 1) + lower) ; }